Method and apparatus for making high strength metals with a face-centered-cubic structure

ABSTRACT

A process for increasing the strength of pure copper and other fcc matrix alloys. The method is particularly applicable to face-centered-cubic materials that undergo dynamic recovery when strain-hardened at room temperature. A cryogenic strain hardening process is used to create a high strength pure copper or copper+Al 2 O 3  alloy. The strength of the material is substantially increased. However, the loss of conductivity is minimal. In the preferred embodiment, pure copper or a copper alloy is drawn into a wire at a temperature of about 77 K. Dynamic recovery of the material is substantially reduced. With this method, drawn copper wire exhibits a strength level about 45% higher than that achievable by an equivalent room temperature deformation.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to the field of materials. More specifically, theinvention comprises a method and apparatus for strengthening metalswhile limiting the dynamic recovery phenomenon.

2. Description of the Related Art

It is possible to strengthen many metallic materials throughwork-hardening (sometimes called “strain hardening”). This isparticularly true for materials having a face-centered cubic (“fcc”)structure. Such materials form atomic matrices, where the basic unit isa cube having an atom located at each of the cube's eight corners and inthe middle of each of the cube's six faces. Stainless steels,superalloys, and aluminum alloys can assume this structure, with ironatoms as the corners and the faces of the cube.

The cubic structure only remains uniform within a particular region,often called a “grain.” Thus, when viewed on a microscopic level, apiece of fcc metal is comprised of many interlocking grains. Within eachof these grains is a relatively uniform fcc matrix.

Plastically deforming a metal introduces dislocations into itscrystalline structure. These generally tend to make the material harderand stronger. A typical example is drawing rod-shaped metal samples intosmaller and smaller wires. The drawing process creates strain hardening.As the strain hardening increases, the ultimate tensile strength (“UTS”)increases. Drawing high-carbon steel in this fashion produces “pianowire,” which has a much higher UTS than unworked steel.

It is generally desirable for a strain-hardened piece of material toremain in the strain-hardened state. This depends upon the retention ofthe dislocations added to the material. For most materials, retention ofthe strain hardening is not an issue. However, some materials experiencea phenomenon known as “dynamic recovery.” When this occurs, addeddislocations are “undone” over time. The material lapses partially orfully back into its undeformed state, thereby destroying the desirableeffects of strain hardening. Copper is a good example of a materialwhich exhibits dynamic recovery.

Those skilled in the art will know that copper and its alloys have beenused for many years in many electrical devices. In the design of suchdevices, one must consider the mechanical strength of the conductingmaterial and its conductivity. These considerations are often inopposition. Pure copper is relatively weak and ductile. It can bestrengthened by adding other alloying elements. However, the addition ofsuch elements reduces its conductivity significantly.

Pure copper can also be strengthened by strain hardening. This processreduces the copper's conductivity moderately. However, the strengthachievable is limited due to the dynamic recovery phenomenon. A methodof strain hardening copper while reducing the effect of the dynamicrecovery phenomenon would therefore be desirable.

BRIEF SUMMARY OF THE PRESENT INVENTION

The present invention comprises a process for increasing the strength ofpure copper and other fcc matrix alloys. The method is particularlyapplicable to face-centered-cubic materials that undergo dynamicrecovery when strain-hardened at room temperature. A cryogenic strainhardening process is used to create a high strength pure copper orcopper+Al₂O₃ alloy. The strength of the material is substantiallyincreased. However, the loss of conductivity is minimal.

In the preferred embodiment, pure copper or a copper alloy is drawn intoa wire at a temperature of about 77 K. Dynamic recovery of the materialis substantially reduced. With this method, drawn copper wire exhibits astrength level about 45% higher than that achievable by an equivalentroom temperature deformation.

The inventive method attains high strength through the stableaccumulation of very high dislocation densities. The work hardening rateis changed by deforming the material under cryogenic conditions. Themethodology can potentially be applied to many different materials whichsuffer dynamic recovery and consequent low strain hardening whendeformed at room temperatures.

The inventive method can also produce highly-aligned dislocations. If,as an example, the dislocations are aligned with the central axis of acopper wire, the dislocations will have a greatly-reduced effect on thewire's conductivity.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS

FIG. 1 is a comparison of strain hardening for two copper materialsdeformed at room temperature.

FIG. 2 is a comparison of the stress-strain curves for the same materialdeformed at room temperature and under cryogenic conditions.

FIG. 3 is a comparison of the strain hardening rates for the samematerial deformed at room temperature and under cryogenic conditions.

FIG. 4 is an elevation section view showing the use of a diameterreducing die to strain harden a material under cryogenic conditions.

FIG. 5 is an elevation view, showing the use of reels to pass materialthrough a diameter reducing die.

FIG. 6 is an elevation view, showing an alternate embodiment of thedevice shown in FIG. 5.

FIG. 7 is a graphical depiction of the grain boundaries within astrain-hardened sample.

FIG. 8 is a graphical depiction of the grain boundaries within astrain-hardened sample made according to the present invention.

REFERENCE NUMERALS IN THE DRAWINGS

10 grain 12 grain boundary 14 wire axis 16 cryostat 18 wire 20cryogenically deformed wire 28 liquid nitrogen 30 deformation zone 32ambient atmosphere 34 draw reel 36 payoff reel 40 idler pulley

DETAILED DESCRIPTION OF THE INVENTION

The present invention contemplates strain hardening materials undercryogenic conditions. Those skilled in the art will know that strainhardening can be produced by virtually any device which plasticallydeforms a sample of material. Strain hardening devices include rollingmills, punches, breaks, roll formers, and drawing dies.

One familiar example of strain hardening is drawing a metal wire througha diameter reducing die (a “wire drawing die”). Such a dies plasticallydeforms the wire to a smaller diameter. A series of such dies canproduce progressively smaller and smaller diameters.

The present inventive method uses conventional strain hardening devices.However, both the strain hardening device and the material being strainhardened are immersed in a bath of cryogenic liquefied gas during thestrain hardening process. This approach is used to reduce the dynamicrecovery phenomenon in face-centered-cubic materials.

The cryogenic deformation process is well suited to the creation ofstrain hardened conductive wires. It can be used to create a highstrength pure copper conductor, a copper alloy conductor made withcopper+Al₂O₃, and other materials.

The copper material is preferably strain hardened at a temperature ofabout 77 K (the approximate temperature of liquid nitrogen boiling atatmospheric pressure). Once the strain hardening is performed, the drawnmaterial is allowed to return to room temperature.

The process has been used for a drawn pure copper wire of having adiameter of 3 mm. The resulting strength was 580 MPa while stillretaining a conductivity of more than 96% IACS (International AnnealedCopper Standard). This strength level is about 45% higher than thatachievable by an equivalent room temperature deformation of copper. Thematerial had a strength level of 680 MPa at 77 K and the resistivityratio was larger than six.

The use of strain hardening at cryogenic temperatures is believed to:(1) produce a stable accumulation of very high dislocation densities;(2) increase the work hardening rate; and (3) alter the microstructure(as will be explained in more detail subsequently).

The process is particularly suitable for strengthening pure copper andcopper matrix materials. These materials can be used as electricalconductors in many areas, such as high field pulsed magnets, highefficiency motors, and medical sensors. The process achieves theincrease in strength without significantly reducing conductivity.

FIG. 1 is a plot of strain hardening characteristics for two coppersamples worked at room temperature. The copper samples are C10C100 andUNS-C15715. C10100 is 99.99% pure copper (by weight). UNS-C15715 is aCu+Al₂O₃ alloy. The unalloyed strength of the copper is increased inUNS-C15715 by the aluminum oxide dispersion in the existing coppermatrix.

Both the pure copper and the aluminum oxide dispersion-strengthenedcopper were strain hardened at room temperature (295 K). For the purecopper (C10100), no further strain hardening occurred after a purestrain of about 1.5. The UNS-C15715 sample can be further strainhardened. Pure copper can be strain hardened at room temperature to astrength level of about 450 MPa. UNS-C157115 can be strain hardened atroom temperature to about 580 MPa.

In order to achieve higher strength one would naturally seek to increasethe deformation strain. However, when this is done at room temperature,the rate of change in the strain hardening (dσ/dε) approaches zero. Thisoccurs because fcc metals (such as copper) exhibit dynamic recoverybeyond a certain amount of strain. For fcc metals strengthened withalloying elements the rate of change in the strain hardening remainsgreater than zero, but it is very small.

FIG. 2 shows the stress-strain curve for UNS-C15760 (another aluminumoxide dispersion-strengthened copper). The lower curve reflects strainhardening conducted at room temperature. The upper curve representsstrain hardening conducted at 77 K. The reader will observe how the roomtemperature curve flattens, while the upper curve continues to increasein strength for increasing strain.

FIG. 3 shows a comparison of the strain hardening rate of UNS-C-15760for strain hardening performed at room temperature versus strainhardening performed at 77 K. The upper curve represents the 77K sampleand the lower curve represents the room temperature sample. The readerwill observe how the strain hardening rate is significantly increasedfor the sample hardened at 77 K.

From FIGS. 1 through 3, one may conclude that the application of strainhardening in a cryogenic environment has significant advantages. If thedeformation is performed at cryogenic temperatures, much higher flowstress levels and a significantly higher strain hardening rate can beachieved.

FIGS. 2 and 3 refer to an aluminum oxide strengthened copper. In theregion above about 800 MPa (for the 77 K sample), dislocationaccumulation occurs due to the presence of the nano-scale aluminaparticles. This accumulation promotes the formation of sub-grains oreven nano-grains in the copper. In-face sub-grains on the scale of 100nm have been observed in the pure copper deformed at 77 K.

At a stress of 800 MPa, the material hardens at a rate of approximately2000 MPa (strain being unit less). This strain hardening rate isapproximately G/16, where G is the shear modulus of copper (about 33GPa). For room temperature strain hardening, the rate is only aboutG/200. Thus, cryogenic deformation produces a drastic change in thematerial properties, with a significant increase in the strain hardeningrate. The result is that a much higher UTS can be obtained using lessstrain.

Impurities introduced into copper often reduce the conductivity of theresulting alloy in comparison to pure copper. Impurities can change theresistivity by a factor of two per 1% of added impurity. Using thepresent cryogenic strain hardening process for pure copper or copperdispersion-strengthened by aluminum oxide only introduces dislocations.It does not produce a dissolution of any second phase, such as thealuminum oxide. Therefore, the decrease in conductivity resulting fromthe cryogenic strain hardening process is mainly due to dislocationaccumulation.

This fact allows the change in resistivity to be used to calculate anestimation for dislocation accumulation. The resistivity of pure coppercryogenically drawn to a strain of 2.3 is 1.79×10⁻⁸Ω·m (at roomtemperature). This figure represents only about a 4% increase over theresistivity of pure annealed copper. Therefore, although the cryogenicdrawing process increases the strength considerably, the increase inresistivity is minimal.

Now that the reader understands the advantages of the cryogenic strainhardening process, devices used to implement the process will bediscussed. The following examples are intended to illustrate the broadconcepts of such devices, with the understanding that many variationsare possible.

FIG. 4 is a sectional elevation view through a cryostat 16 containingliquid nitrogen 28. The term “cryostat” will be understood to encompassany vessel capable of containing a cryogenic liquefied gas. Vesselsusing a Dewar-type evacuated wall construction work well in thisapplication, but other constructions are possible.

The liquid nitrogen is vented to the atmosphere, resulting in themaintenance of its boiling temperature of about 77 K. An fcc metal isplaced in the cryostat along with the strain-producing device. In theparticular example shown, a copper or copper alloy wire 18 of a firstdiameter is placed within the vessel. It is forced through a diameterreducing die 22 which draws it down to a second reduced diameter as thewire passes through deformation zone 30. Cryogenically deformed wire 20remains in the liquid nitrogen for some period before exiting thecryostat and returning to ambient atmosphere 32. Thus, the strainhardening occurs at cryogenic temperatures.

Those skilled in the art will realize that the device illustrated inFIG. 4 is not very practical. Wire drawing operations generally involvelong lengths of wire, requiring the use of drums or spools to maintaindesirable tension. FIG. 5 shows an elevation view of a moresophisticated device. A liquid nitrogen cryostat 38 is used to housepayoff reel 36. It pays out copper wire of a first diameter. The wire isdrawn through a wire drawing die 22 as before (The die is also immersedin the liquefied gas contained within the cryostat. The wire is thentaken up on the rotating draw reel 34, which also applies tension.

The top of the cryostat is depicted as being open, though in reality itwould need to be mostly sealed in order to prevent an inordinately highboil-off rate. The drawn wire would likely exit the cryostat through asmall opening through a well insulated wall.

FIG. 6 shows an alternate embodiment in which the payoff reel and drawblock are both located outside the cryostat. Two submerged “idler”pulleys 40 are preferably used to maintain tension on the wire as itpasses through the cryostat. However, the larger payoff reel and drawreel remain outside the cryostat. This fact means that the cryostat canbe smaller and that the payoff reel and draw reel do not need to bedesigned to withstand the extremely low temperatures found in thecryostat. As for the embodiment of FIG. 5, the top of the cryostat shownin FIG. 6 would actually be closed with only small openings provided forthe passage of the wire.

FIGS. 7 and 8 illustrate some of the grain properties of the deformedfcc materials and serve to explain some of the advantages obtained byperforming the present inventive method. FIG. 7 shows a graphicaldepiction of the grain boundaries in a strain-hardened fcc metal. Theimage is a cross sectional view, with the section being transverse tothe direction in which the metal was drawn. The left hand view shows asample which was drawn at room temperature (“RT Sample”), whereas theright hand view shows a sample which was drawn under cryogenicconditions (“LNT Sample”) using the present process.

The RT Sample shows elongated cells, which are a result of dynamicrecovery after the strain hardening. The LNT Sample, in contrast, showscells having a generally circular cross section. A cell size for the LNTSample is about 150 nm, whereas a cell size for the RT Sample is about190 nm. The LNT Sample shows more variation in cell size, which again isa result of the reduction in the dynamic recovery phenomenon. Suchmicrostructure differences contribute to the property differencesbetween the two types of samples.

FIG. 8 shows another graphical depiction of the grain structure in anfcc metal sample, with the section this time being taken in the sameplane as the direction in which the metal was drawn. The reader willobserve how the dislocations are highly oriented, generally aligningwith the direction of drawing.

Although the preceding description contains significant detail, itshould not be construed as limiting the scope of the invention butrather as providing illustrations of the preferred embodiments of theinvention. As an example, although copper and copper alloys have beendiscussed in the descriptions, the inventive method could be applied toother fcc metals which exhibit dynamic recovery. The inventive processcould likewise be carried out in many different ways. As an additionalexample, sheet material could be fed through a small rolling mill (whichwould reduce the sheet thickness and increase its width) contained in acryostat. The rolling mill would be the strain producing device andwould be equivalent to the drawing dies shown in the illustrations.Thus, the scope of the invention should be fixed by the following claimsrather than the examples given. The inventors acknowledge the helpfuldiscussion provided by Dr. J. D. Embury.

Having described our invention, we claim:

1. A method for strain hardening a length of metal while reducingdynamic recovery, comprising: a. providing a vessel containing acryogenic liquefied gas; b. providing a deformation die, located withinsaid vessel and immersed in said cryogenic liquefied gas; and c. passingsaid length of metal through said deformation die while a portion ofsaid length of metal proximate said deformation die and said deformationdie remain immersed in said cryogenic liquefied gas.
 2. A method ofstrain hardening as recited in claim 1, wherein said cryogenic liquefiedgas is liquid nitrogen.
 3. A method of strain hardening as recited inclaim 1 wherein said length of metal is a wire and said deformation dieis a diameter reducing die.
 4. A method of strain hardening as recitedin claim 3, wherein said wire is fed from a payoff reel, through saiddiameter reducing die, and onto a draw reel.
 5. A method of strainhardening as recited in claim 4, wherein said payoff reel is containedwithin said vessel and said draw reel is located outside said vessel. 6.A method of strain hardening as recited in claim 4, wherein both saidpayoff reel and said draw reel are located outside said vessel.
 7. Amethod of strain hardening as recited in claim 6, wherein said wire isfed from said payoff reel to a first idler pulley, then through saiddiameter reducing die, then to a second idler pulley, and then to saiddraw reel.
 8. A method of strain hardening as recited in claim 7,wherein said first and second idler pulleys are contained within saidvessel.
 9. A method for strain hardening a length of metal whilereducing dynamic recovery, comprising: a. providing a volume ofcryogenic liquefied gas; b. providing a deformation die; c. immersingsaid deformation die in said volume of cryogenic liquefied gas; and d.using said deformation die to deform said length of metal while aportion of said length of metal proximate said deformation die and saiddeformation die remain immersed in said cryogenic liquefied gas.
 10. Amethod of strain hardening as recited in claim 9, wherein said cryogenicliquefied gas is liquid nitrogen.
 11. A method of strain hardening asrecited in claim 9 wherein said length of metal is a wire and saiddeformation die is a diameter reducing die.
 12. A method of strainhardening as recited in claim 11, wherein said wire is fed from a payoffreel, through said diameter reducing die, and onto a draw reel.
 13. Amethod of strain hardening as recited in claim 12, wherein said payoffreel is contained within said vessel and said draw reel is locatedoutside said vessel.
 14. A method of strain hardening as recited inclaim 12, wherein both said payoff reel and said draw reel are locatedoutside said vessel.
 15. A method of strain hardening as recited inclaim 14, wherein said wire is fed from said payoff reel to a firstidler pulley, then through said diameter reducing die, then to a secondidler pulley, and then to said draw reel.
 16. A method of strainhardening as recited in claim 15, wherein said first and second idlerpulleys are contained within said vessel.
 17. A method of strainhardening as recited in claim 3, wherein said cryogenic liquefied gas isliquid nitrogen.
 18. A method of strain hardening as recited in claim 4,wherein said cryogenic liquefied gas is liquid nitrogen.
 19. A method ofstrain hardening as recited in claim 11, wherein said cryogenicliquefied gas is liquid nitrogen.
 20. A method of strain hardening asrecited in claim 12, wherein said cryogenic liquefied gas is liquidnitrogen.